1. Field of the Invention
The present invention generally relates to vehicle detection. More particularly, this invention pertains to portable or temporary sensors and related deployment and analysis methods used for the detection, classification, or re-identification of automotive vehicles.
2. Description of the Related Art
Collection of real-time traffic data is useful for work-zone safety, Advanced Traveler Information Systems (ATIS), Advanced Transportation Management Systems (ATMS), traffic law-enforcement, and for collision avoidance among many other things.
Collection of historical traffic-flow data is essential for making well informed infrastructure planning decisions, and for validating and calibrating sophisticated traffic flow and econometric models. Prior-art methods for collecting wide area historical traffic-flow data have not provided as much data as is needed on a cost effective basis, and they can require significant disruption of traffic flow while sometimes exposing staff to unnecessary risk. The limitations of prior-art data collection methods have spurred the development of products that use roadside-deployable technologies to detect traffic. The most successful of these prior-art technologies are side-fire RADAR, and Video Image Processing Systems (VIPS); however, RADAR systems are limited in their ability to monitor multi-lane traffic and they are not adequate for precision vehicle re-identification. VIPS include License Plate Recognition (LPR) systems as well as vehicle shape/color recognition systems. VIPS systems perform reasonably well (˜98% accuracy) when the lighting and weather conditions are favorable, but are privacy-intrusive, are not reliable for round-the-clock operations, suffer from occlusion, and are difficult to calibrate in place without a reference detection system.
Comprehensive information on the use of transportation facilities provides the basis for many of the decisions made regarding the transportation infrastructure. Generally, the traffic data needed to support the decision-making process and the design process includes traffic volume (vehicle counts), vehicle classification (typically by axle count), average speeds, and lane occupancy. Travel-time and origin/destination (O/D) data is particularly useful to planners, but it has been notoriously difficult, expensive, and privacy-intrusive to the public to collect this data in the past. The availability and reliability of the traffic data collected for use by planners is important because it affects funding priorities and the design of highway projects. Yet, until the last decade, the methods for collecting historical traffic data over a wide area were essentially limited to a mixture of fixed counting locations using common inductive loop detectors, common road tube counts, and human observation. Each of these methods has limitations that have historically made traffic data collection a significant challenge, especially in urban areas.
Fixed counting locations with common inductive loop detectors can provide a baseline for traffic data collection. Common road tubes are widely used for temporary sampling of traffic volumes, but they can present problems for staff safety, traffic disruption, and poor data collection performance. Staff safety is a concern when common road tubes must be set where traffic volumes are high during peak periods and relatively high during off-peak periods. Disruption of traffic flow typically occurs when setting common road tubes on moderate or high-volume roadways because temporary closure of traffic lanes may be needed to provide safety for personnel. Performance of common road tube counters is often hampered by complex roadway geo-metrics, multiple lane roadways, and adverse weather conditions.
Manual counts present safety and operational problems. Manual counts can place staff at risk if they must be exposed to vehicular traffic for long periods during counts. Another safety problem results from personnel being located in areas where crime presents a threat to personal safety. Extreme weather conditions further limit the implementation of a conventional manual count. Also, in some cases, the presence of counting staff can affect the traffic flow on very high-volume roadways.
These problems have resulted in a number of new technologies being employed in devices for collecting traffic data in urban areas. These technologies are considered to be non-intrusive because they can be deployed without the need to close lanes to traffic or to expose staff to unsafe conditions. Even though traffic detection devices using these non-intrusive technologies have been available for several years, there are still many uncertainties regarding their appropriate application and performance.
The following factors must be considered when evaluating non-intrusive devices: Level of expertise required and time spent installing and calibrating a device; Reliability of a device; Number of lanes a device can detect; Mounting options such as overhead, side-fire and height; Ease of installation and moving from one location to another; Capability for remote adjustment of calibration parameters and trouble shooting; Wireless communication to simplify the data retrieval process; Solar powered or battery powered devices for temporary counts in locations without an accessible source of power; Type of traffic data provided; Performance in various weather and traffic conditions; and the intended use for a particular device, (e.g., a device used to actuate a signal must meet a different set of performance criteria than a device used to collect historical traffic data). Some devices are also designed to offer real time information for ITS applications.
Many of these non-intrusive devices are well suited for temporary counting situations. Ease of installation and flexibility in mounting locations and power supplies are important elements in selecting a portable device that can be installed quickly and moved from location to location. The devices that use Doppler microwave, active infrared, and passive infrared technologies have a simple “point-and-shoot” type of setup. Passive magnetic, radar, passive acoustic and pulse ultrasonic devices require some type of adjustment once the device is mounted. In most cases this adjustment must be performed over a serial communication line. Video devices require extensive calibration over serial communication lines and are not well suited for temporary counting. Extensive installation work is required for video and passive magnetic devices, making them less suitable for temporary data collection. From an overhead mounting location at the freeway test site, the video and passive acoustic devices have been found to count within four to ten percent of baseline volume data. Pulse ultrasonic, Doppler microwave, radar, passive magnetic, passive infrared, and active infrared have been found to count within three percent of baseline volume data. The count results are more varied at intersection test sites. The pulse ultrasonic, passive acoustic, and video devices are generally within ten percent of baseline volume data while some passive infrared devices can perform within five percent. Speed data can be collected from active infrared, passive magnetic, radar, Doppler microwave, passive acoustic, and video devices. In general, all of these devices can measure speed within eight percent of baseline. Radar, Doppler microwave, and video are the most accurate prior-art technologies at measuring vehicle speeds. Video and radar devices have the advantage of multiple-lane detection from a single unit. Video has the additional advantage of providing a view of the traffic operations. Weather variables have been found to have minimal direct affect on device performance, but snow on the roadway can cause some vehicles to track outside of their normal driving patterns, affecting devices with narrow detection zones. Lighting conditions have been observed to affect some of the video devices, particularly in the transition from day to night. Extremely cold weather can make access to such devices difficult, especially for the magnetic probes installed under the pavement. Urban traffic conditions, including heavy congestion, have been found to have little effect on the performance of these devices. In general, the differences in performance from one device to another within the same technology have been found to be more significant than the differences from one technology to another. Among the various technologies of the prior-art, it may be more important to select a well designed and highly reliable product than to narrow a selection to a particular technology.
Available prior-art devices are known to incorporate multiple technologies within a single device. Developments in other technologies, such as passive millimeter microwave and infrared video, are expected to produce additional entries into the market.
Conventional wheel-spike detectors of the prior art are typically implemented as axle detectors using pneumatic tubes, but are occasionally implemented using piezoelectric strips, filter optic treadle, or narrow-aperture inductive loops. Pneumatic tubes are widely used for temporary traffic counts, and have demonstrated a modest capability for vehicle classification.